The Effects of Duty Ratio and Intensity of Pulsed LED Light on Growth and Photosynthetic Rate of Lettuce Grown in a Plant Factory System

This study was carried out to examine the effect of duty ratio and intensity of pulsed LED light on growth and photosynthetic rate of lettuce (Lactuca sativa L. ‘Cheongchima’ and ’Jeokchima’) grown in a plant factory system. The 21-day old seedlings were grown for four weeks under the bar type LEDs (5 red:2 blue:1 white) with different LED pulse (on/off) of 400/0 (continuous), 300/100, 200/200, 133/266, and 100/300 μs by using oscilloscope during 16 h photoperiod. These indicated to duty ratio (DR, percentage of on time from one cycle) of 100%, 75%, 50%, 33, and 25%, respectively. Light intensities were two levels (high and low, HL and LL) at DR 100%, 75%, and 50%. Air temperature and relative humidity in the system were maintained at 20 ± 2 °C and 70% ± 10%, respectively. Nutrient solution (initial pH 5.8 ± 0.2, EC 1.2 dS·m^-1, 20 ± 2 °C) was supplied by nutrient film technique (NFT) methods. After two weeks of treatment, fresh and dry weights per plant in ‘Cheongchima’ lettuce were no significant difference in DR 100% HL, 75% HL, 50% HL, and 33%. The number of leaves of ‘Cheongchima’ was no significant difference in DR 100% LL, 33% HI. Fresh and dry weights of ‘Jeokchima’ were no significant difference in DR 100% HL and 75% HL. The Hunter’s ‘a’ values were significant difference among treatments. After four weeks of treatment, fresh weight of ‘Cheongchima’ was no significant difference in DR 100% HL and LL, 75% HL, and 50% HL. The dry weight per plant of ‘Cheongchima’ was no significant difference in DR 100% HL and LL, 75% HL and LL, and 50% HL. These results will be utilized for energy saving in the plant factory system using LED as a light source.